In the cathodes of the type described above, the aluminium plate is usually terminated along its upper edge in a bar which is thicker and longer than said edge. With this arrangement, the cathodes are situated in the electrolytic tanks with the aluminium plate in the vertical position where the bar acts as a head for supporting the cathode in the tank and for connecting the lifting apparatus which handles it.
During the process of electrolysis, the zinc is deposited progressively over the entire surface of the aluminium plate in the region which is submerged in the electrolytic tank.
Generally, the vertical edges of the aluminium cathode plate are coated with a protective dielectric material which prevents the zinc from being deposited on the vertical edges. In this way, the layers of zinc deposited on each surface of the aluminium plate are prevented from joining along the vertical edges of said plate.
The layer of zinc deposited on the cathodes adheres to the surface of the aluminium plate. The devices used to detach this layer include two vertically moving knives terminating in converging horizontal lower edges, angular in section and of approximately the same length as the width of the cathodes. To use these devices for detaching the plates of zinc from the cathode, one knife is situated on each side thereof, resting and pressing the edges of both knives against said surfaces, above the height reached by the deposits of zinc on the cathode. The knives are then moved downwards such that when their edges meet the layer of zinc deposited on each side of the cathode they carry out its separation.
This system has the drawback that knives erode the surfaces of the cathode over its entire height, making it unusable after a certain number of operations. Due to the high cost of the cathodes, this system of detaching the zinc is, therefore, inadequate.
An attempt to solve the above mentioned problems has been made by means of a system which includes two successive phases of action on the cathodes. During a first phase, the upper edge of the deposited layers of zinc are detached by means of a horizontally acting lateral penetrator, and during a second phase, the layers of zinc are pulled free entirely by means of two vertical knives. The lateral penetrator consists of two horizontal arms which have an acting end whose edge is in the form of a vertical angle wedge; said arms being able to move axially, one on each side of the cathode at the height of the upper edge of the deposited layer of zinc. Whilst these arms move, the acting end thereof presses against the surface of the cathode and causes the erosion and deterioration thereof.
In order to help the action of the lateral penetrator, the British patent No. 1,326,418 describes cathodes on which a pivoting piece made of a dielectric material is mounted on the upper part of one of the vertical edges, situated at the height of impingement and action of the penetrator, which corresponds to the height on the plate reached by the electrolyte in the tank. This piece is channelled and grips the cathode plate to which it is fixed by means of an axis which is perpendicular to said plate. The piece pivots about said axis between a lower position, in which it is coupled to and covers a certain region of the plate, as a prolongation of the protective profile of the vertical edges, preventing the zinc from being deposited thereon during the process of electrolysis, and an upper position in which said region is free and uncovered, enabling the arms of the lateral penetrator to move up to the surfaces of the cathode plate in this region and, as it continues to advance, meeting the edge of the zinc deposit in order to detach it. These cathodes have one fundamental drawback which derives from the need to rotate the pivoting piece from its lower position to its upper position each time that the zinc deposits are to be detached. As the cathode is introduced into the tank, the pivoting piece should occupy its lower position. The rotation of the pivoting piece and the conditions to which said piece is subjected cause the adjustment between the piece and the cathode, and fundamentally between the axis of rotation, to be lost and deteriorate in a relatively short period of time; the pivoting piece having to be raised by hand during the operation of detaching the zinc, with the corresponding increase in cost of the process in the form of labour and increase in the corrosion of the cathode sheet in this region.
The German patent DE-A-3051150 describes a cathode whose plate has a fixed coating made of an insulating material starting from one of the vertical edges, in the region where the lateral penetrators impinge and rest. This coating extends over regions which coincide with the surfaces of the plate and over the section of the intermediate vertical edge. The external surfaces of the coating are flat and coplanar with those of the plate, for which both surfaces of said plate are provided with undercuttings where the shape and depth of which determine the shape and thickness of the coating of insulating material for each side of the plate. To prevent this coating from separating from the cathode plate, the region in which the undercuttings for fixing said coating are made may be provided with through holes via which the coating layers on each of the surfaces of the plate are joined. This constitution has manufacturing problems due to the need to make the lowered regions on both surfaces of the cathode plate and the through holes between said regions and via which the coatings of both surfaces have to be joined. All of this implies a considerable increase in the cost of manufacture of the cathodes. Furthermore, due to the action of the lateral penetrators on the insulating coating, when they begin to act on the cathode, they cause the premature breaking of the joins between the coatings of each side, causing them to separate from the plate. Furthermore, the thickness of the insulating coatings is relatively reduced, and the action of the penetrators causes their premature deterioration, exposing part of the plate and thereby giving rise to irregular deposits of zinc which obstruct the action of the penetrators. This makes it necessary to carry out periodic revisions of the cathodes in order to replace the insulating coatings.